Have you asked if they are willing to come out a DIFFERENT weekend so that you can schedule it in a way that is actually convenient for ALL of you?

I wouldn't schedule it at a time that doesn't really work for YOU just because that is the only time they are willing to do it. You will be silently fuming the whole time and it will be a big black mark on the whole experience.

As the PP said, you have a few options including picking other godparents, using a proxy, or just a whole different weekend altogether. Don't just settle for a date/time that doesn't work for you just to fit their schedule and make her happy.

But don't expect her to bend over backwards to accommodate your baptism timing either. The wedding is the reason they are coming out, I wouldn't expect them to compromise that to satisfy you.

Sounds like from you're post you piggybacked the baptism off of their travel plans on a wedding they'd already committed to and planned to attend. If that is the case then you play 2nd fiddle IMO, and work around their schedule.